Magnetic excitation and hole doping effects of Haldane system

点击翻译按钮获取中文摘要

After the quantum fluctuations in Haldane system have been observed, a lot of research has been performed both from theoretical and experimental sides. Antiferromagnetism can be induced by impurity doping in the non-magnetic gapped Haldane system. In this impurity doped Haldane system, antiferromagnetism and gapped Haldane state coexist, resulting in the magnetic new state.We tried to observe the magnetic excitations in the carrier doped Haldane system, paying attention to the carrier (hole) dependence. In order to observe the detailed physical properties with neutron scattering experiments, Nd_2BaNiO_5 was chosen for which a large-sized single crystal can be available. The Haldane gap in Nd_2BaNiO_5 has shown the similar behavior to that of pure Haldane system Y_2BaNiO_5 above T_N. The Haldane state coexists with antiferromagnetic state below T_N, which was recently recognized as a unique model of Haldane gap quantum spin chain in a staggered applied field.We clearly observed the dynamical incommensurate structure above T_N in the hole doped (Nd, Ca)_2BaNiO_5. The incommensurability δ is not proportional to the hole concentration. This corresponds to the relationship between the hole concentration and δ in the copper oxide superconductor. A new magnetic dispersionless excitation was observed with hole doping below T_N, although this excitation energy is higher than the energy currently expected. In order to observe this magnetic excitation, we try to use the ESR technique having a better energy resolution. Since we succeeded in growing a large single crystal of (Nd, Y, Ca)_2BaNiO_5 for which we can control the influence of magnetic Nd ion freely, we will perform the neutron experiment in near future..